The subject of the present invention is a method for stabilizing a hydrocarbon-conversion catalyst by substantially precluding contact between the catalyst and water during the interval from the calcination of the finished catalyst to the use of the catalyst for the conversion of hydrocarbons.
Catalysts having a cracking function and a hydrogenation-dehydrogenation function are used in a variety of hydrocarbon-conversion applications to accelerate a wide spectrum of reactions, particularly in the petroleum and petrochemical industries. The cracking function generally is thought to be associated with an acid-action material of the porous, adsorptive, refractory-oxide type which is typically utilized as the support or carrier for a heavy-metal component, such as the Group VIII (8-10) metals, to which is generally attributed the hydrogenation-dehydrogenation function.
These catalysts are used to accelerate a wide variety of hydrocarbon-conversion reactions such as dehydrogenation, hydrogenation, hydrocracking, hydrogenolysis, isomerization, desulfurization, cyclization, alkylation, polymerization, cracking, and hydroisomerization. In many cases, the commercial applications of these catalysts are in processes where more than one of these reactions are proceeding simultaneously. An example of this type of process is reforming wherein a hydrocarbon feed stream containing paraffins and naphthenes is subjected to conditions which promote dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization of paraffins and naphthenes, hydrocracking of naphthenes and paraffins and other reactions to produce an octane-rich or aromatic-rich product stream. Another example is an isomerization process wherein a hydrocarbon fraction which is relatively rich in straight-chain paraffin compounds is contacted with a dual-function catalyst to produce a product stream rich in isoparaffin compounds while converting any cyclics present to a mixture of paraffins and naphthenes by a combination of hydrogenation and ring opening. Yet another example is a hydrocracking process wherein catalysts of this type are utilized to effect selective hydrogenation and cracking of high-molecular-weight unsaturated materials, selective hydrocracking of high-molecular-weight compounds, and other reactions to produce a generally lower-boiling, more-valuable product stream.
Regardless of the reactions or the particular process involved, it is of critical importance that the dual-function catalyst exhibit the capability both to initially perform its specified functions efficiently and to perform them satisfactorily for prolonged periods of time. The parameters used in the art to measure how well a particular catalyst performs its intended functions in a particular hydrocarbon reaction environment are activity, selectivity and stability. In a reforming environment, these parameters are defined as follows:
(1) Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level, with severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Activity in a catalytic reforming process typically is designated as the octane number of the pentanes and heavier ("C.sub.5 +") product stream from a given feedstock at a given severity level, or conversely as the temperature required to achieve a given octane number.
(2) Selectivity refers to the precentage yield of desired product, e.g., for reforming the C.sub.5 + gasoline or petrochemical aromatics product from a given feedstock at a particular activity level.
(3) Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured in the case of reforming as the rate of change of operating temperature per unit of time or of feedstock to achieve a given C.sub.5 + product octane, with a lower rate of temperature change corresponding to better activity stability, since catalytic reforming units typically operate at relatively constant product octane. Selectivity stability is measured as the rate of decrease of C.sub.5 + product or aromatics yield per unit of time or of feedstock.
Programs to improve reforming-catalyst performance are being stimulated by the widespread removal of lead antiknock additive from gasoline and by the increasing requirements of high-performance internal-combustion engines, which magnify the requirement for gasoline "octane" or knock resistance of the gasoline component. The catalytic reforming unit must operate at higher severity in order to meet these increased octane needs. This higher severity has a highly leveraged effect in catalyst stability, decreasing yield and increasing the temperature required to maintain product "octane." The surpising improvement in catalyst stability when the present method is utilized to substantially preclude contact between the catalyst and water during the interval from the calcination of the finished catalyst through the startup of the process unit has not heretofore been recognized.